Optical disk recording systems, where a large capacity of recording can be achieved and contactless and high-speed access can be performed, have been developed to use as a large capacity memory. Such optical disks are classified into a read-only type such as a compact disk and a laser disk, a recordable-type whose user can record information and a rewritable type whose user can repeatedly record and erase information. The recordable-type and rewritable-type optical disks are used as an external memory of computers or a document and image file. The read-only type, which is rapidly popularized as a data file such as CD-ROM, is used as a high-density recording medium for personal use. Also, due to its large capacity, CD-ROM has been researched to use as a multimedia file including image data by using an image compressing technology such as MPEG2. For this use, a capacity of 650 MB/disk in present CD-ROMs is not sufficient, and a capacity four to eight times this capacity is desired. On the other hand, read-only type image files such as a laser disk have been standardized to contain high-quality high vision images in a CD size so as to provide DVD products. However, its recording time is not sufficient when considering a picture quality. Thus, also in this type, a higher-density medium is desired.
The recordable optical disk has got a certain market in the application field where its merit that recorded information can be safely stored is utilized maximumly. Also in this type, a higher-capacity and higher-density medium is desired.
In the rewritable-type optical disks, there are included a phase-change type optical disk where a phase change of recording film is used and a magneto-optical disk where a change of magnetization direction in perpendicular magnetization film is used. The phase-change type optical disk does not need an external magnetizing device and can be easily overwritten. Therefore, it is expected that this type will be predominant in the rewritable-type optical disks as well as the magneto-optical type. In magneto-optical disks, a product with a capacity four times that of first generation magneto-optical disks has been researched, while it is reported that a capacity eight or more times that is technically possible (for example, Honma et al., SPIE, Vol.2338, p.314). Also in phase-change type optical disks, a product standard corresponding to the four-times capacity of magneto-optical disks and a test product development where an application to small-size large-capacity video disk recorder is considered( for example, Okubo et al., 7th Phase Change Optical Recording Symposium, preliminary papers, p.48, and Matsui et al., 7th Phase Change Optical Recording Symposium, preliminary papers, p.54) are suggested. Also in this type, large capacity and high density are important keywords.
In the read-only type, metal reflection film made of Al alloy system is formed on concave-convex pits formed previously on a substrate by injection molding and is used for reproducing. In the recordable type, used is a medium where an alloy composed of low melting point metal such as Te, Bi, Se and Sn or a dye material with an absorption characteristic at a laser wavelength used is thin coated on a substrate. In the rewritable type, magneto-optical disks have a medium composition where alloy thin film composed of rare earth metal such as Tb, Gd, Dy and Ho and transition metal such as Fe, Co and Ni is sandwiched by transparent protective coating film made of, e.g., SiN, and phase-change optical disks have recording film made of chalcogenide such as GeSbTe and InSbTe. Also, for the recording film of the phase-change optical disks, InSe system, InTe system, AsTeGe system, TeOx-GeSn system, TeSeSn system, SbSeBi system and BiSeGe system materials are used. Meanwhile, the above thin films are formed by resistance heating evaporation, electron beam evaporation, sputtering or spin coating.
In the above read-only type, recordable-type and rewritable-type optical disks, concave-convex pits, recordable-type recording film, magneto-optical recording film or phase-change recording film is formed on a rigid substrate. In the read-only type, a signal is continuously reproduced by tracking the concave-convex pits themselves. In the recordable type and rewritable type, recording and reproducing are conducted by using a guide groove for tracking which is previously formed on the rigid substrate. Thus, the conventional optical disks use the substrate on which the guide groove for tracking convergent laser beams are previously formed to record, reproduce and erase information. A most effective way for conducting high-density recording in such optical disks is to increase the track density. Therefore, to narrow the width of the guide groove as well as to shorten the wavelength of laser light to be used has been researched. For example, it is considered to use a substrate with a track pitch of 0.9 to 1.2 .mu.m for laser light with a wavelength of 685 nm, while a substrate with a track pitch of 1.3 to 1.6 .mu.m has been used for laser light with a wavelength of 830 nm.
In such optical disks, narrower tracks are desired for higher-density recording. However, when the track width is so narrowed that the tracking groove width is less than 1/2 of laser wavelength to be used, it becomes very difficult for laser light to enter inside the groove since the oscillation mode of light to be allowed between both ends of the groove wall is limited. Therefore, the tracking characteristic is highly deteriorated, and it becomes very difficult to record a signal into the groove and to reproduce a signal therefrom. Thus, there is a big problem that it is substantially impossible to use such a narrowed track substrate.